So I'm starting to have the same problems that my first character had, with my newest character.
Her Crafts are in the 30s, and she's max rank with the GC and only has a piddly cap of 50k seals. Woo.
The trouble with that, is one single HQ turn-in is worth 800+ seals at that level, starred items can get up around 2K, and I find myself having to dump a bunch of seals every day I turn GC stuff in. I want the craft/gatherer XP, but I don't really need the seals, but I don't want to waste them either. Now, I know I've a few options, but...
1). Ventures. Yeah, 1000 seals for 5 ventures. That's good and all, except I have 500+ of them and this number is climbing rapidly. I'm working on my 6th stack of the stupid things. Also, the Ixals love giving me ventures too. Heck, I can sustain myself infinitely almost on Ixal ventures alone assuming 18-h explorations once a day.
2). Dark Matter. Except... I already have 95+ of all 5 grades of it. I Suppose I could build up a 2nd stack of G5 for S&G, but meh.
3). Cordials. Treasure Chests (the ones you get by doing treasure maps) seem to love dropping these; I've got like 20 of them and I simply don't need that many cordials.
4). Lv50+ Crafting Materials. Being as I'm not Lv50 in any crafts yet (BSM is getting close at 45!), and I don't know what I need or how much of it I need, I don't want to buy a ton of crap only to have to somehow store it (I already have storage problems now because of how much crap exists in the game, even just mob drops). Oh, and I can use Oaknots to buy them too if I so desired. In fact, that's the only things Oaknots can buy that I saw, so I'd obviously use those first before using GC seals.
5). Chocobo Barding. Already bought all of these.
6). Furniture. I never bothered with buying personal quarters or living space, I'm not THAT rich.
So... outside of maybe XP manuals (the only other job I was planning on leveling in any "soon" timespan was BLM), I can't find anything to spend the GC seals on. They set the cap extremely low, but even if they had raised the cap, I woulda just capped out a few days later, even if they set it to something like 100k. The problem is, we need more options on consumables, things we actually care about that isn't already available in mass quantities elsewhere.
For example, I'd love to have a GC->Gil conversion. Maybe 2-3GC seals for 1 gil? (333-500gil per 1000 GC seals sounds fair). That'd at least be something I can get out of my GC seals. There's Bird respecs, but I don't need a respec for my bird. The weapons and equipment, I have all the stuff I care about/need at the moment.
Or how about Teleport Tickets? Maybe 1k GC Seals for 5 Teleport Tickets. That way, anytime I'm going to take a 300g+ teleport, I can take it for free. But I'd eventually fill up on those too, lol. It'd at least give me something else I can spend GC seals on.